从源码的角度分析setDescendantFocusability三种值的含义

由
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  1. FOCUS_BEFORE_DESCENDANTS ViewGroup本身先对焦点进行处理，如果没有处理则分发给child View进行处理
  2. FOCUS_AFTER_DESCENDANTS 先分发给Child View进行处理，如果所有的Child View都没有处理，则自己再处理
  3. FOCUS_BLOCK_DESCENDANTS ViewGroup本身进行处理，不管是否处理成功，都不会分发给ChildView进行处理

通过ViewGroup的源码可以得出上面的结论：

@Override
public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
    if (DBG) {
        System.out.println(this + " ViewGroup.requestFocus direction="
                + direction);
    }
    int descendantFocusability = getDescendantFocusability();

    switch (descendantFocusability) {
        case FOCUS_BLOCK_DESCENDANTS:
            //ViewGroup本身进行处理，不管是否处理成功，都不会分发给ChildView进行处理
            return super.requestFocus(direction, previouslyFocusedRect);
        case FOCUS_BEFORE_DESCENDANTS: {
            //ViewGroup本身先对焦点进行处理，如果没有处理则分发给child View进行处理
            final boolean took = super.requestFocus(direction, previouslyFocusedRect);
            return took ? took : onRequestFocusInDescendants(direction, previouslyFocusedRect);
        }
        case FOCUS_AFTER_DESCENDANTS: {
            //先分发给Child View进行处理，如果所有的Child View都没有处理，则自己再处理
            final boolean took = onRequestFocusInDescendants(direction, previouslyFocusedRect);
            return took ? took : super.requestFocus(direction, previouslyFocusedRect);
        }
        default:
            throw new IllegalStateException("descendant focusability must be "
                    + "one of FOCUS_BEFORE_DESCENDANTS, FOCUS_AFTER_DESCENDANTS, FOCUS_BLOCK_DESCENDANTS "
                    + "but is " + descendantFocusability);
    }
}
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